Active noise control (ANC) (also known as noise cancellation, active noise reduction (ANR) or antinoise) is a method for reducing unwanted sound.
The basic elements of an active noise control system include one or more microphones to sense the noise over some location of interest and a means to produce an appropriately tailored sound field that acts as a “control field.” The control field is usually created through the action of a control system connected to loudspeakers. Applications of the concept have included control of noise produced by industrial fans, suppression of noise in heating, ventilation, and air conditioning ducts, reduction of vehicle exhaust sounds, creation of “quiet zones” within vehicle interiors, reduction of the noise levels inside aircraft and spacecraft launch fairings, and the now almost commonplace active noise control headsets in aviation and consumer use.
Active noise control dates back to 1934. U.S. Pat. No. 2,043,416, incorporated by reference herein, describes a process of silencing sound oscillations by receiving sound oscillations which are to be silenced, and reproducing them in the form of sounds having an opposite phase.
Sound is a pressure wave, which consists of a compression phase and a rarefaction phase. A noise-cancellation speaker emits a sound wave with the same amplitude and the opposite polarity (in antiphase) to the original sound. The waves combine to form a new wave, in a process called interference, and effectively cancel each other out—an effect which is called phase cancellation. Depending on the circumstances and the method used, the resulting soundwave may be so faint as to be inaudible to human ears.
The human ear can nominally hear sounds in the range 20 Hz to 20,000 Hz (20 kHz). This upper limit tends to decrease with age, most adults being unable to hear above 16 kHz. The ear itself does not usually respond to frequencies below 20 Hz, but these can be perceived via the body's sense of touch. The “intensity” range of audible sounds is enormous. Human ear drums are reportedly sensitive only to the sound pressure variation. The lower limit of audibility is defined as 0 dB (decibels), but the upper limit is not as clearly defined. The upper limit is more a question of the limit where the ear will be physically harmed or with the potential to cause a hearing disability. This limit depends also on the time of exposure to the sound. The human ear can be exposed to short periods in excess of 120 dB without permanent harm—albeit with discomfort and possibly pain; but long term exposure to sound levels over 80 dB can cause permanent hearing loss.
Noise at all, and tonal noise in particular is very annoying. In many cases, the noise level may collide with safeness standards and in radical situations may harm health.
Noise can be classified into several categories related to its spectrum shape, namely the distribution of the acoustic energy over the audible frequency range (which is typically 20-20,000 Hz).
A “tonal” noise is noise with its energy concentrated at a few specific frequencies. A whistle is a radical example for a tonal noise in which the acoustic energy concentrates at a single frequency, producing a single tone noise. Complex products containing several types of fans and/or engines which usually produce a significant whistle may produce tonal noise in which the acoustic energy concentrates at several separable frequencies.
A “broadband” noise is noise with its energy spread over a range of frequencies, but not necessarily the whole audible range (20-20,000 Hz). An air-conditioner noise is an example of a broadband noise in which its energy spreads on the range of 100-1500 Hz. Air-flow at-all is a typical example of a broadband noise.
In most cases, the spectrum of the noise contains a tonal noise and a broadband noise, when in a spectrum curve the tonal noise peaks protrude above the broadband noise spectrum curve. A noise measured within a car is a typical example of such a noise including tonal and broadband ingredients.
A noise-cancellation speaker may be co-located with the sound source to be attenuated. In this case it must have the same audio power level as the source of the unwanted sound. Alternatively, the transducer emitting the cancellation signal may be located at the location where sound attenuation is wanted (for example, the user's ear). This requires a much lower power level for cancellation but is effective only for a single user. Noise cancellation at other locations is more difficult as the three dimensional wavefronts of the unwanted sound and the cancellation signal could match and create alternating zones of constructive and destructive interference. In small enclosed spaces (for example, the passenger compartment of a car) such global cancellation can be achieved via multiple speakers and feedback microphones, and measurement of the modal responses of the enclosure.
Modern active noise control is achieved through the use of a computer, which analyzes the waveform of the background aural or nonaural noise, then generates a polarization reversed waveform to cancel it out by interference. This waveform has identical or directly proportional amplitude to the waveform of the original noise, but its polarity is reversed. This creates the destructive interference that reduces the amplitude of the perceived noise.
ANC methods differ from passive noise control methods (soundproofing) in that a powered system is involved, rather than unpowered methods such as insulation, sound-absorbing ceiling tiles or muffler.
Sound absorption materials for use in passive noise control may range from mineral wool and glass fiber blankets to open cell foams made of polyurethane, polyimide and melamine, having a variety of surface treatments. The effectiveness of all these materials in acting as sound absorbers or isolators tends to be highly frequency dependent.
Regarding passive noise control, generally, elastic porous materials such as polyurethane foam reduce noise by dissipating the energy of the incident sound wave, using the friction related to the coupling of the solid constituting the frame and the fluid (air) in the holes. The method of passive noise control using the elastic porous material is the method to absorb sounds merely by installing the elastic porous material between the noise origin and the sound receipt point. This method is simple in structure and easy to implement. Thus, this method has been widely used because of the low cost incurred in installing the device and the broad frequency bandwidth in which noise may be reduced. However, this method has a drawback in that it can hardly control noises of low frequencies.
Generally, passive noise control methods, such as foam, are most effective above a frequency such as 1000 Hertz (1 KHz). And active noise control methods are most effective at lower frequencies.
The passive means, which are in use in current products, are not efficient in the low frequencies and particularly when dealing with fan noise in which one should not block or impair the airflow. The noise emitted by standard fans in routine use is characterized by one or several tones at the low frequencies range (<1000 Hz). These tones generally cannot be reduced by means of passive treatment under limitation of weight and size. For example, in order to reduce a tone at 500 Hz in about 10 dBA would expectedly require the use of a muffler of more than 1 meter length and 30 cm diameter. Active noise control technology is designed to overcome this disadvantage, and may commonly achieve 25 dBA reduction of separated tones, and 10 dBA in a range of frequencies noise.
Exemplary ANC Systems
A basic feed-forward active noise control system generally consists of a reference sensor (such as a microphone), an electronic controller, a loudspeaker and an error sensor (such as a microphone). The reference microphone picks up the information of the primary noise field and sends it to the electronic controller; the controller then drives the control loudspeaker to radiate the antinoise; the error microphone examines the control performance and modulates the controller for the best result.
An example of an active noise control system and method may be found in commonly-owned Patent Publication No. WO 2005/027338 (“338”), incorporated by reference herein. As shown and described therein, an active noise control (ANC) system may include an acoustic sensor (typically a microphone) to sense the noise energy and/or wave amplitude of a noise pattern produced by a noise source. The ANC system may also include an acoustic transducer (for example a speaker), and a controller to control the speaker to produce a noise destructive pattern to reduce or cancel the noise energy and/or wave amplitude of the noise pattern, for example within a reduced-noise zone. The controller may include an estimator to produce a predicted noise signal by applying an estimation function to one or more samples of noise signal. A noise error signal may be sensed by a second acoustic sensor (error microphone) positioned in the reduced-noise zone.
Digital adaptive reduction of noise in the time domain is typically performed by sampling the analog output of a microphone that is appropriately positioned to sense the input noise. The sampled analog noise is then converted to digital format via an A/D (analog-to-digital) converter, passed through an adaptive digital filter and then converted back to analog via a D/A (digital-to-analog) converter before being output to a speaker. The analog output of a microphone is utilized as the input to the internal adaptive algorithm within the prior art noise reduction system.
A method of noise cancellation used in prior art systems places the microphone as close to the noise source as possible and the loudspeaker relatively far from the microphone so as to create a delay equal to the time for the noise to travel from the microphone to the speaker. This delay is intentionally created in order to match the internal signal processing time of the noise reduction system. The propagation time for the noise is configured to roughly match and compensate for the signal propagation time within the noise reduction system. This noise reduction method is particularly useful for cancellation of noise in a duct such as an air conditioning duct. The internal signal processing is performed during the time that it takes for the sound waves to travel from the microphone to the loudspeaker.
Virtual Microphone
It is understood to generate what is referred to as a “virtual microphone” to eliminate the need for a second “error microphone”. The technique is based on a previously measured acoustic transfer function between an existing microphone, usually the reference microphone, a microphone, the virtual microphone, temporarily placed at a position further away from the existing one and the speaker input. Applying the transfer function between the reference microphone and the virtual microphone to the reference microphone signal, yields an estimation of the source noise at the virtual microphone location. Applying the transfer function between the speaker input and the virtual microphone to the speaker input signal, yields an estimation of the destruction signal at the virtual microphone location. Summing up (adding together) the two estimated signals yields an estimation of the error signal or the residual noise.
Types of Fans
A fan is an example of a device used to induce airflow and is generally made from broad, flat surfaces which revolve or oscillate. There are three main types of fans used for moving air: axial fan (see FIGS. 1A and 1B), centrifugal fan also called radial fan (see FIG. 2) and cross flow fan also called tangential fan (see FIG. 3). Axial (or axial-flow) fans have blades that force air to move parallel to a mandrill (or axle) about which the blades rotate. Axial fans blow air across the axis of the fan, linearly, hence their name. The centrifugal fan has a moving component (called an impeller) that consists of a central shaft about which a set of blades form a spiral pattern. Centrifugal fans blow air at right angles to the intake of the fan and spin (centrifugally) the air outwards to the outlet. Tangential fans take in air along the periphery of the rotor and expel it through the outlet in a similar fashion to the centrifugal fan. Cross-flow fans give off an even airflow along the entire width of the fan, and are very quiet in operation. They are comparatively bulky, and the air pressure is low.
FIGS. 1A and 1B illustrate an axial-flow fan having an impeller comprising a plurality of vanes (or blades). The vanes extend radially from a hub, and are all generally in a plane. The fan has a circular housing. A motor may be connected to the hub for rotating the hub, hence the vanes, about an axis. When the vanes are rotating, air flows generally parallel to the axis, from an inlet side of the fan, axially, through the vanes, to an outlet side of the fan, where the flow is also axial, substantially collinear with the inlet flow. The fan may alternatively have a square-shaped housing, as indicated by the dashed lines in FIG. 1A, may be a “muffin fan”, and may alternatively have the motor incorporated into the hub (not shown).
FIG. 2 illustrates a centrifugal flow fan having an impeller comprising a plurality of vanes (or blades). The vanes extend generally axially at a radial distance from an axis of rotation, and there is an open space inside the vanes. The fan has a generally spiral shaped housing. A motor may be connected to the impeller for rotating the vanes, about the axis. When the vanes are rotating, air flows from an inlet side of the fan, generally axially to a space inside the vanes, through the vanes, to an outlet side of the fan where the flow is more-or-less radial, at approximately 90 degrees to the inlet flow.
FIG. 3 illustrates a tangential flow fan having an impeller comprising a plurality of vanes (or blades). The vanes extend generally axially at a radial distance from an axis of rotation, similar to the centrifugal fan, but are generally longer than the vanes of the centrifugal fan, and there is not the open space inside the vanes that there is in the centrifugal fan. The fan has a generally spiral shaped housing. A right (as viewed) end of the housing is shown broken away, to better see the impeller and vanes. A motor (not shown) may be connected to the impeller for rotating the vanes, about the axis. When the vanes are rotating, air flows from an inlet side of the fan, radially, through the vanes, to an outlet side of the fan where the flow is also radial at approximately 90 degrees to the inlet flow.
Noise Concerns
An undesirable side effect of the induced airflow is annoying noise which stems from the mechanical friction of the fan components such as the fan motor, the rotor mandrill and the friction between the blades and the air, and also from the air movement. Since the shell of the fan has some sort of noise obstruction capabilities, and since in all kinds of fans, the airflow is directional in the intake as well as in the outtake, a fan can be regarded as a bi-directional noise source in which the two major noise sources are the inlet (intake) and the outlet (outtake, exhaust).
The standard “JIS B 8330” (Testing Methods for Turbo-Fans, Japanese Standards Association, Publication Date: Jul. 20, 2000) is commonly used as a guideline for fan noise measurements. However, due to these standard and other common measurements methods, the fan noise is measured in a so-called free field, in which no load is acting on the fan. The measurements result is a parameter that usually is published in a dedicated column in the fan specifications.
In most applications, the fan is not applied as a stand-alone device, and is usually coupled with another mechanical device such as a heat sink, duct or radiator, that imposes a significant load on the fan and may significantly increase the fan noise. And, once installation effects are taken into account, then the actual levels will typically exceed the free field ones, due to reflections off adjacent surfaces, such as floors and walls. Actual installations will be somewhere between a free field and reverberant environments.
Traditional Solutions
Traditional methods to ease the acoustic problem such as mufflers, sealing or absorbing materials may appear to be ineffectual since the nature of the fan as an airflow generator compels at least one direct contact between the fan core and the ambience. As noise is better travel through air the noise can be emitted through this direct contact, which eliminates the sealing solution or other solutions based on blocking the noise by acoustic barriers.
Mufflers or silencers are not effective also since mufflers usually contain a series of baffles to absorb sound, although the majority of the noise reduction is not through absorption but through destructive interference in the muffler itself. The muffler accomplishes this with a resonating chamber, which is specifically designed such that opposite sound waves are likely to collide, canceling each other out. The set of baffles and chambers presents a significant resistance to air, and may reduce airflow such that the original goal of the fan can not be achieved.
Absorbing materials may be a partial solution only since in most fans with dimensions exceeding 60×60 mm, the dominant noise frequencies are below 1 kHz, in which the absorbing materials are almost useless.
Feed-Forward And Feedback Systems
Feed-forward is a term describing a kind of system which reacts to changes in its environment, usually to maintain some desired state of the system. A system which exhibits feed-forward behavior responds to a measured disturbance in a pre-defined way, as contrasted with a feedback system.
Many prerequisites are needed to implement a feed-forward control scheme: the disturbance must be measurable, the effect of the disturbance to the output of the system must be known, and the time it takes for the disturbance to affect the output must be longer than the time it takes the feed-forward controller to affect the output. If these conditions are met, feed-forward can be tuned to be extremely effective.
Feed-forward control can respond more quickly to known and measurable kinds of disturbances, but cannot do much with novel disturbances. Feedback control deals with any deviation from desired system behavior, but requires the system's measured variable (output) to react to the disturbance in order to notice the deviation.
A feed-forward system can be illustrated by comparing it with a familiar feedback system—that of cruise control in a car. When in use, the cruise control enables a car to maintain a steady road speed. When an uphill stretch of road is encountered, the car slows down below the set speed; this speed error causes the engine throttle to be opened further, bringing the car back to its original speed.
A feed-forward system on the other hand would in some way ‘predict’ the slowing down of the car. For example it could measure the slope of the road and, upon encountering a hill, would open up the throttle by a certain amount, anticipating the extra load. The car does not have to slow down at all for the correction to come into play.
Sound Power Level
Sound Power and Sound Pressure are two distinct and commonly confused characteristics of sound. Both share the same unit of measure, the decibel (dB), and the term “sound level” is commonly substituted for each. Sound power is the acoustical energy emitted by the sound source, and is an absolute value—the environment does not affect it. Sound power levels are connected to the sound source and independent of distance of measurements.
Sound PowerSound PressureSound Power is the amount ofSound Pressure is the pressureacoustic energy being generateddeviation from the local ambientper unit time by the source.pressure caused by a sound wave.Sound Power cannot be measuredSound Pressure can be measureddirectly.with a microphone.Sound Power is not affected bySound Pressure is affected bythe environment.the environment.Lp=LW+10 log [(D/4Πr2)+(4/R)]Π
where                Lp is the Sound Pressure level in dB        LW is the Sound Power level in dB        D is the directivity factor        Π is “pi” (3.14)        R is the environment constant in m2 sabins        r is the radius of the measurements        
Loudspeakers (Speakers)
Conventional loudspeakers, independent of the method of transduction they use (electromagnetic, electrostatic, piezoelectric), aim at achieving pistonic motion of the diaphragm. “Pistonic” movement means a displacement of the diaphragm as a rigid whole. In acoustic terms, such a loudspeaker is mass-controlled. For a given input voltage the motor generates a force that is constant with frequency, the diaphragm resists with its mass according to Newton's second law of motion. To avoid acoustical shortcut between the front and backside of the diaphragm the loudspeaker benefits from an enclosure. For powerful sound radiation the volume has to be huge in order to minimize the pressure differential within the enclosure while the diaphragm is displaced. Additionally, the huge cavity may lead to additional problems with resonances.
Another kind of sound source uses pneumatic energy as the actuator. Ships' sirens, for instance, with rotating interrupter disks are ideally suited to the reproduction of loud and far-reaching sound mixtures, but only for repetitive signals. Then there are also ‘air-modulated devices’. In these, a compressed air stream is modulated through an electrodynamically actuated valve.
Whereas a conventional loudspeaker functions with large membrane areas and small vibration amplitudes, with pumps the same volume flow and therefore the same acoustic emissions can be achieved, but with a very much smaller sound radiating opening area. The Air Pump Speaker uses micro- and nanomechanical pumps for air, gas or fluids. Thanks to their small dimensions, these pump systems have very short reaction times and are therefore capable of delivering the output flow necessary for generating a powerful sound signal. See, for example, Patent Publication No. WO 00/47012, sound generator with a pump actuator, incorporated by reference herein.